One of the essential points of a gas-phase fluorination process is the stability of the catalyst.
Several solutions have been suggested to maintain the stability of the catalyst.
Thus, document EP 609123 describes a continuous process for catalytic fluorination of perchloroethylene in the gas phase by means of hydrofluoric acid in the presence of a mixed catalyst composed of nickel oxides, halides and/or oxyhalides and chromium oxides, halides and/or oxyhalides deposited onto a support consisting of aluminum fluoride or of a mixture of aluminum fluoride and alumina.
The stability of the catalyst is demonstrated in example 2 of this document, with a temperature of 350° C., at atmospheric pressure, an HE/PER ratio in the region of 7 and a contact time of 15 seconds.
During the fluorination of perchloroethylene by means of hydrofluoric acid in the presence of a catalyst, a mixture of compounds forms, with predominantly the compounds of the “F 120 series”, namely F 121 (CHCl2-CCl2F), F 122 (CHCl2-CClF2), F 123 (CHCl2-CF3), F 124 (CHFCl—CF3) and F 125 (CHF2-CF3), or isomers thereof. In addition to the compounds of the “F 120 series”, the mixture contains in particular F 115 (CF3-CF2Cl), F 114a (CF3-CFCl2), F 114 (CF2Cl—CF2Cl), F 133a (CH2Cl—CF3) and olefins F 1111 (CFCl═CCl2) and F 1112a (CF2═CCl2).
When the process is carried out under conditions where the conversion of the perchloroethylene (PER) is not complete, it is essential to recycle the unreacted PER in order to obtain a process for producing pentafluoroethane which is economically competitive. However, the recycling of the reactants is often accompanied by deactivation of the catalyst.
It has now been found that a pretreatment, before the recycling, of the stream leaving the reaction step makes it possible to maintain the stability of the catalyst.